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You are here: Home / Circuits & Projects / How To Build An Automatic Night Light Circuit

How To Build An Automatic Night Light Circuit

March 13, 2020 By Øyvind Nydal Dahl 33 Comments

In this tutorial, you’ll learn how to build an automatic night light circuit that turns on when it gets dark. It’s a simple circuit that you can build on a breadboard.

This circuit shows you how to do it with an LED. But you can use the same principle to turn on bigger and brighter lights too.

Find the breadboard diagram and parts list below the video.

The Components You’ll Need

Night light circuit diagram
  • 9V Battery
  • Breadboard
  • Photoresistor (around 5kΩ in light, 200kΩ or more in dark)
  • Transistor BC547
  • Resistor 100 kΩ
  • Resistor 470 Ω
  • Light-Emitting Diode (LED)

There are many ways to connect this circuit. I recommend using a breadboard since it’s quick and you can easily reuse components.

Below you can see how I connected this circuit on a breadboard:

Night light circuit on breadboard

How The Night Light Circuit Works

The photoresistor and the 100 kΩ resistor make up a voltage divider.

When there is a lot of light, the photoresistor will have low resistance, which means the voltage divider gives a low output voltage. So the transistor is off and cuts off the current to the LED. Which means no light.

When it’s dark, the photoresistor will have high resistance. That means the voltage divider gives a high output voltage which turns on the transistor. That means the LED is also on and will light up.

The finished night light circuit

What Are the Voltages out From the Voltage Divider?

When it’s light and the photoresistor value is low, the output from the voltage divider is around 0.5V, which is not enough to turn on the transistor.

When it’s dark and the photoresistor value is high, the output from an unconnected voltage divider would be around 4.5V.

But since the output of the voltage divider is connected on the base of the transistor, the voltage will be limited by the forward voltage of the base-emitter connection (around 0.7V).

Questions?

Did you build the night light circuit? Do you have any questions about how it works or how to build it? Let me know in the comment section below.

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More Circuits & Projects Tutorials

  • Basic Circuits
    • Blinking Light Circuit
    • Automatic Night Light
    • Touch Sensor
    • Light-Dependent Resistor (LDR) Circuit
    • Sunrise Wake-Up Alarm
  • Games & Fun
    • The Knight Rider LED Bar
    • Baseball Game with 5-Run Display
    • DIY Sugar Dispenser
    • Halloween Project: Jack-O-Lantern
    • Halloween Project: The Useless Machine
    • 5-Minutes Christmas Tree Ornament
    • Blinking Christmas Lights
  • Sound
    • The Simple Circuit That Makes Sound
    • The Atari Punk Console
    • How to Build an Arduino Theremin
    • Electronics Project: 1937 radio hacked into an iPhone dock
    • A Simple Amplifier Circuit
    • A Simple Mono Amplifier Circuit
  • Power Supply
    • The Simplest Power Supply Circuit
    • How to Make a Dual 5V Power Supply From USB
    • Portable USB Charger Circuit
    • Portable USB Charger Components And Tools
  • Misc Projects
    • Digital Stopwatch Circuit
    • DIY Sous Vide Cooker
    • Recyclomat – the Rube Goldberg Recycling Machine
    • Radar Circuit Examples
    • Simple VU-Meter Circuit
    • How To Measure Water Tank Depth Using Ultrasonic Sensor
    • Timer Project with Gong
    • Plant Moisture Sensor Prototype
    • Ho, ho ho! 5 Christmas Circuit Ideas
    • Sound Card Oscilloscope
    • Intel Galileo: Color-Changing Lamp
    • Intel Galileo: DIY Weather Station
    • Intel Galileo: Boot From an SD-card
    • Intel Galileo: Measuring Rain and Moist
    • Intel Galileo: Installing Whatsapp
  • Circuit Ideas And Where To Find Them
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